1. Field of the Invention
The present invention relates in general to an anti-lock or anti-skid braking system of a motor vehicle, and more particularly to techniques for suitably controlling braking pressures for rear right and left wheels of the vehicle.
2. Discussion of the Related Art
There is known a braking system of diagonal or X-crossing type, which is used for four-wheel motor vehicle having front right and left wheels and rear right and left wheels. This diagonal type braking system has two pressure application sub-systems connected to respective two mutually independent fluid pressurizing chambers of a master cylinder. One of the sub-systems includes a front left wheel brake cylinder and a rear right wheel brake cylinder, while the other sub-system includes a front right wheel brake cylinder and a rear left wheel brake cylinder. Each pressure application sub-system further includes a reservoir, a pump for pressurizing the fluid received from the reservoir, and at least one solenoid-operated valve device each of which is disposed in a fluid passage connected to at least one of the pressurizing chambers, wheel brake cylinders, reservoir and pump, so that the fluid pressure in each wheel brake cylinder is controlled. The pressure application sub-system is constructed such that a decrease in the fluid pressure in the front wheel brake cylinder will necessarily cause a decrease in the fluid pressure in the rear wheel brake cylinder. A controller is provided to control each solenoid-operated valve device for suitably controlling the fluid pressure (I.e., braking pressure) in each of the wheel brake cylinders, in an anti-lock fashion so as to prevent an excessive amount of slip of each wheel during braking of the vehicle.
An example of an anti-lock braking system of the type described above is disclosed in U.S. Pat. No. 5,538,334 to Kushi et al., which corresponds to JP-A-7-223529.
In the braking system disclosed in the above-identified U.S. Patent, each of the two pressure application sub-systems has a front brake cylinder passage connecting one of the pressurizing chambers of the master cylinder to the front wheel brake cylinder, and a rear brake cylinder passage which is connected at one end thereof to the front brake cylinder passage and at the other end to the rear wheel brake cylinder. A first solenoid-operated valve device is disposed in a portion of the front brake cylinder passage between the master cylinder and a point of connection between the front and rear brake cylinder passages, and a second solenoid-operated valve device is disposed in the rear brake cylinder passage. Further, a reservoir passage is connected at one end thereof to a portion of the rear brake cylinder passage between the second solenoid-operated valve device and the rear wheel brake cylinder, and at the other end to the reservoir. A third solenoid-operated valve device is disposed in this reservoir passage. In addition, a pump passage is connected at one end thereof to the reservoir, and at the other end to the front brake cylinder passage or a portion of the rear brake cylinder passage between the second solenoid-operated valve device and the point of connection to the front brake cylinder passage. The pump is disposed in the pump passage. To lower the braking pressure in the front wheel brake cylinder in each pressure application sub-system of this braking system, both of the second and third solenoid-operated valve devices must be simultaneously opened for fluid communication of the front wheel brake cylinder with the reservoir. As a result, the braking pressure in the rear wheel brake cylinder of the same pressure application sub-system is necessarily communicated with the reservoir and is consequently lowered.
In the braking system of the type described above, the controller generally includes control mode selecting means and solenoid valve control means for controlling the braking pressures in the brake cylinders of the rear right and left wheels. The control mode selecting means is adapted to select one of a plurality of pressure control modes, for each rear wheel brake cylinder. The pressure control modes include a pressure decrease mode, a pressure hold mode and a pressure increase mode. The solenoid valve control means is adapted to control at least one solenoid-operated valve device so as to establish the selected pressure control mode for each of the rear wheels.
The braking pressures in the brake cylinders of the rear right and left wheels are usually controlled according to a so-called "Select Low Control", which is formulated to assure a high level of running stability of the vehicle during braking on a road surface which has different values of friction coefficient in its right and left areas on which the right and left wheels are running. According to the "Select Low Control", the same pressure control mode is established for both of the rear right and left wheels, even if the different pressure control modes are normally selected according to a normal "Independent Control" in which the pressure control modes for the two rear wheels are selected independently of each other. That is, where the pressure control modes normally selected for the rear right and left wheels are different from each other, one of these two control modes which causes a large amount of decrease of the rear braking pressure than the other control mode is used for both of the rear wheels. This "Select Low Control" of the rear wheel braking pressures results in a decrease in the rear braking forces, but assures an increase in the maximum lateral forces acting on the rear wheels, making it possible to reduce lateral movements of the rear portion of the vehicle during braking. That is, the vehicle running stability during braking is improved at the sacrifice of the rear braking forces, according to the "Select Low Control" of the rear braking pressures.
Described more specifically, if the pressure decrease mode or pressure hold mode is normally selected for one of the rear right and left wheels according to the "Independent Control" while the pressure increase mode is normally selected for the other rear wheel according to the "Independent Control", the braking pressure for this other rear wheel is also controlled in the pressure decrease or hold mode selected for the above-indicated one rear wheel. In this case, the braking pressures of the two rear wheels are lowered or remain unchanged. Similarly, if the pressure decrease mode is normally selected for one of the rear wheels while the pressure hold mode is normally selected for the other rear wheel, the braking pressure for the other rear wheel is also controlled in the pressure decrease mode selected for the above-indicated one rear wheel. In this case, the braking pressures of the two rear wheels are lowered.
As a result of research on a vehicle anti-lock braking system of diagonal type adapted to control the braking pressures of the rear wheel brake cylinders according to the "Select Low Control", the present inventor recognized a need of improvement in such a braking system regarding the control of the rear wheel brake cylinder pressures.
As described above, the "Select Low" control of the braking pressures of the rear wheels aims at improving the running stability of the vehicle during braking, at the sacrifice of the rear braking forces. To attain a satisfactory level of compromise between the reduction of the required braking distance of the vehicle and the running stability, it is important to prevent an excessively large amount of reduction of the rear braking forces. In this respect, it is noted that the braking system of the diagonal or X-crossing type is constructed such that the braking pressure in the rear wheel in each pressure application sub-system is necessarily reduced upon reduction of the braking pressure in the front wheel in the same sub-system, as described above, even when the reduction of the rear wheel braking pressure is not required. Therefore, the practicing the "Select Low Control" as originally intended may cause an unnecessarily large amount of reduction of the rear wheel brake cylinder pressure, and an accordingly large amount of reduction of the rear wheel braking forces, which is unnecessary to assure a satisfactory degree of the vehicle running stability during braking.